This invention relates to a catalyst, catalyst precursor, and catalyst carrier of a particular cross sectional shape for use in catalysing reactions, particularly mass transfer limited reactions, such as Fischer-Tropsch reactions or hydrocracking reactions.
The Fischer-Tropsch process can be used for the conversion of synthesis gas (from hydrocarbonaceous feed stocks) into liquid and/or solid hydrocarbons. Generally, the feed stock (e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into one or more reactors where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into mainly paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more. Preferably the amount of C5+ hydrocarbons produced is maximized and the amount of methane and carbon dioxide is minimized.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidized bed reactors, such as entrained fluidized bed reactors and fixed fluidized bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
In the past work has been devoted to the development of particles, in particular catalytically active particles, for many different processes. There has also been a considerable effort to try to understand the advantages and sometimes disadvantages of effects of shape when deviating from conventional shapes such as pellets, rods, spheres and cylinders for use in catalytic as well as non-catalytic duties. Examples of further known shapes are rings, cloverleafs, dumbbells and C-shaped particles.
Particles that can be made using extrusion, pelletizing and pressure moulding have been the subject of various studies, especially particles suitable for use in (multi tubular) fixed bed reactors. Considerable efforts have been devoted to the so-called “polylobal”-shaped particles. Many commercial catalysts are available in TL (Trilobe) or QL (Quadrulobe) form. They serve as alternatives to the conventional cylindrical shape and often provide advantages because of their increased surface-to-volume ratio, which results in a smaller effective particle size, thus providing a more active catalyst.
A variety of shapes and designs of catalyst particles for use in the fixed bed operation of the Fischer-Tropsch synthesis have been proposed. EP 428 223 describes catalyst particles in the form of hollow cylinders, for example cylinders having a central hollow space which has a radius of between 0.1 and 0.4 of the radius of the cylindrical extrudate, and rifled (or twisted) trilobes. Trilobe extrudates are said to be preferred.
When using a process employing a fixed bed of catalyst particles, a major consideration in the design of the process is the diffusion limitations of the catalyst. Different reactants will typically travel through the catalyst at different rates. Thus the surface area of the catalyst is preferably maximized to minimize diffusion limitation.
In addition to the above, the catalyst particles should be sufficiently strong to avoid undesired attrition and/or breakage. Especially in fixed beds the bulk crush strength should be (very) high, as beds are used in commercial reactors of up to 15 meters high. Especially at the lower end of the bed the strength of the catalyst particles plays an important part. The handling of catalyst before entry into the reactor also requires the catalyst to have reasonable strength.
However, providing a catalyst shape with a sufficient strength but with maximum surface area are somewhat conflicting requirements which complicates the design of further improved catalyst particles.
Furthermore, there is a need for a fast, relatively inexpensive and suitable manufacturing process which will enable the production of catalyst particles in large quantities.
To produce a strong extrudate, a trilobe catalyst with a ‘cloverleaf’ cross section can be made. Examples of such trilobes have been described in, for example, U.S. Pat. No. 3,857,780 and U.S. Pat. No. 3,966,644. Trilobe catalysts with a ‘cloverleaf’ cross section are sometimes referred to as “TL” shaped catalysts. FIG. 1 shows a perspective view of one embodiment of such a trilobe shape. A trilobe catalyst with a ‘cloverleaf’ cross section shows a good mechanical strength but the mass transfer limitations are considered too restrictive. Especially for Fisher Tropsch reactions and hydrocracking reactions the mass transfer limitations of such trilobe catalysts are too restrictive.
WO 03/103833 describes catalyst particles with a relatively low diffusion limitation. This document discloses an elongated shaped extrudate comprising three protrusions each extending from and attached to a central position. The cross-section of the particle can be defined by considering a central circle surrounded by six outer circles. Each of the six outer circles touches two neighbouring circles. Of the six outer circles, alternate circles are occupied by the particle's cross-section. The cross-section of the particle occupies the space encompassed by the outer edges of the six outer circles, minus the space occupied by the three remaining outer circles. The six interstitial regions, formed by the inclusions of the central circle and six times two adjacent outer circles, are also occupied by the extrudate.
The shape disclosed in WO 03/103833 is sometimes referred to as an “extended trilobe” and sometimes as “Tx” shaped catalyst. FIG. 2 shows the cross-section of one embodiment of such a shape. The cross-section of the particle has a central circle 10 and outer occupied circles 12. Interstitial areas 18 are also occupied by the cross-section of particle 1, which is shown in bold outline. The outer circles 14 are unoccupied.
In WO 03/103833 is indicated that preferred catalyst particles have a cross-section in which the three alternating circles have diameters in the range between 0.74 and 1.3 times the diameter of the central circle. There may be a distance between the three alternating circles and the central circle. If there is any overlap between the three alternating circles and the central circle it will be less than 5% of the area of the central circle. Preferably the catalyst particles have a cross-section in which the three alternating circles are attached to the central circle.
Catalyst particles according to WO 03/103833 show a relatively small amount of the unwanted mass transfer limitations but the shape has been found to suffer from poor strength, making handling outside the reactor difficult.